The present invention is directed to a cam ring for actuating retractable pins for a golf ball injection mold.
It is standard practice in the fabrication of an intermediate layer or cover layer of a golf ball to utilize an injection mold having two mold plates with hemispherical cavities that mate to form a spherical shape when the mold halves are joined. At the initial stage of the injection molding process, a golf ball core placed inside the mold is supported centrally within the mold by a plurality of retractable pins located near the upper and lower poles of the mold cavity so as to leave a space for forming an intermediate or cover layer about the core. The retractable pins are activated by a plate that controls movement of the pins in a vertical direction to engage with the core to hold it securely in place. After the pins have engaged with the core of the ball, thermoplastic or thermnosetting material then is injected into the mold cavity in a horizontal plane from a primary supply through a plurality of edge gates that usually are evenly distributed near or around the parting line of the mold halves and the equator of the inner hemispherical surface of the golf ball. The retractable pins hold the core in place while the injected material fills the void between the core and the inside wall of the mold. Trapped air and gasses escape through vents located at the upper and lower poles of the ball as flow from injected material from each of the plurality of gates eventually joins to fill the void between the golf ball core and the mold. Once the void is nearly filled but before the injected material has completely hardened, the pins are retracted from the mold cavity in a vertical direction until the faces of the pins form a portion of the mold cavity wall. If the pins are retracted only after the injected covering material has contacted the pins, any voids formed from retraction of the pins are filled by the injected material. Once the injected material has substantially hardened, the mold is opened and the ball is removed.
Use of a plurality of retractable pins to securely position the golf ball core during the injection process is known to cause wear at the interface between the surfaces of the pins and the surfaces of the pin holes in the mold plate through which the pins are inserted. Typically, the face of the pins that contact the core of the ball are not normal, i.e. perpendicular, to the direction of the axial force applied to the pins to cause them to engage with the core. In a conventional retractable pin golf ball injection mold, illustrated in FIGS. 1 and 2, the pins are arranged in a circle and are engaged with the core by being inserted into the mold in a vertical direction. The faces of the pins, however, are angled so that they contact the core essentially along a tangent to the contacted surface of the ball in order to have a good grip on the golf ball core. For forming intermediate layers, the faces of the pins that contact the ball core typically are curved to conform with the mold cavity wall (e.g. having a radius cut) or have an angle cut matching the tangent to the point of ball contact, whereas the tips of pins in an injection mold to form a cover layer are shaped with a dimple radius formed on the end thereof. In both instances, the faces of the pins contact the ball at an angle not normal to the direction of the vertical axial force applied to the pins. As a result, the ball core applies a counterbalancing force on the pins that has an axial load component and a cantilever load component. As the pins move under this cantilever load when engaging or disengaging from the ball core, the pin holes are worn out of round and the pins may spread, flex and/or experience extensive wear. In some instances, galling of the pin and pin hole may result. Wear between the pins and pin holes eventually becomes excessive and allows injected material to flow into the worn area, causing undesirable flash on the surface of the molded layer of the ball. The result of this undesired wear is that the manufactured balls require additional process steps to remove the flash and the mold must be shut down periodically for inspection, repair and/or replacement of worn tooling. Thus, it would be desirable to engage and disengage the retractable pins in a direction that is essentially normal to the tangent of the contacted surface of the ball.
A second disadvantage of conventional retractable pin injection molding that results from having the retractable pins not operate in a direction normal to the tangent of the point of contact with the ball core is the time, expense and precision required for forming the face of the pin. Because the faces of conventional retractable pins are angled to better grip the ball, forming a dimple radius on the end of each pin requires expensive, high-precision processing, such as using an EDM process to xe2x80x9cburnxe2x80x9d the shape of the dimple on the end of the pin or using a compound angular set up to produce an oblique conical radius on the pin. The dimple formed on the face of the pin is elliptical because the face of the pin is elliptical due to its angled cut, pins being generally circular in cross-section. Noticeable cosmetic defects can result if the elliptical dimples formed on the face of the pin are even slightly out of place. Moreover, because each pin is custom-made to match the geometry of the mold, the pins can not be used in a mold having a different geometry. Thus, the retractable pins can not be reused or repositioned should the mold geometry or dimple pattern change.
Yet another disadvantage associated with conventional injection molding is that the use of multiple gates dispersed around the equator of the mold cavity is known to cause xe2x80x9cknit linesxe2x80x9d on the newly formed ball layer when injected layer material from neighboring gates intersects as the material fills the mold cavity. xe2x80x9cKnit linesxe2x80x9d are seams along the newly formed intermediate layer or cover layer that are formed where the injected material intermixes from different gates during the formation of the layer.
FIG. 2 illustrates the formation of knit lines 10 as flow from any one gate 12 intersects with flow from a neighboring gate. When a golf ball cover is formed by a conventional retractable pin injection process with multiple edge gates to inject a layer material into a mold, the injected material from each gate has a flow front that eventually intersects with layer material entering the mold from other edge gates. Knit lines are formed at the intersection of each of these converging flow fronts. The multiple knit lines of the newly formed layer ultimately intersect at the flow terminus of the layer material near the upper and lower poles of the mold cavity. As such, there are a number of knit lines or flow fronts throughout a layer where layer material from various gates flows together as it fills the void between the golf ball core and the mold. Depending on the composition of the injected material, the material tensile strength can be reduced by as much as 10% to 60% along the knit lines. Thus, because the intermediate or cover layer is inherently weaker along the knit lines, it is desirable to minimize the occurrence of knit lines when forming a golf ball layer. Therefore, there exists a need for a method of making golf ball layers by an injection molding process that does not result in the occurrence of knit lines, thereby increasing the durability of the layer and extending the useful life of the golf ball.
In addition to resulting in knit lines that may weaken the golf ball cover, conventional multiple edge gate injection molding also may not maintain balanced flow or uniform filling of thermoplastic resin blend cover material between the core and the inside wall of the mold, which may further weaken the golf ball cover. For example, non-uniform filling can cause the flow terminus of the cover to not meet at the poles of the ball where trapped air and gasses typically are released through a vent. When the flow terminus is not at the poles of the mold, the trapped air and gasses cannot evacuate the cavity effectively, thereby further compromising knit line integrity and reducing the durability of the cover.
Even in instances where flow of injected layer material is properly balanced, venting of trapped air and gasses at the poles of the mold can be a limiting factor in the speed at which layer material is injected and eventually requires time-consuming periodic maintenance of the vents to clear clogs. Because it is desirable for the gap between the vent pin and the wall of the mold surrounding the pin to be sufficiently narrow to prevent injected material from escaping and causing flash on the newly formed layer, the limited space provided for trapped air and gasses to escape may slow the injection of layer material. In addition, injected material can become clogged in the vent over time. Removing this trapped debris clogging the vent pin requires periodic maintenance, possibly even requiring disassembly of the mold tooling. Placing vents at the parting line of the mold rather than at the pole would greatly improve venting and allow for easy removal of debris each time the mold is opened to remove the ball.
One solution to eliminating knit lines, achieving balanced flow and improving venting is to use a hot runner system to inject layer material at the poles of the ball rather than using multiple gates at or near the parting line of the mold and to vent trapped air and gasses at or near the parting line of the mold instead of at the poles. The relatively cramped space at the poles of conventional retractable pin injection molds, however, does not permit the use of such a hot runner system at the poles of the ball. Typically, as shown in FIGS. 1 and 2, a plurality of retractable pins 16 surround the pole in part to minimize the angle between the vertical direction in which the pins move and the angle of the faces of the pins contacting the ball, which in turn minimizes wear at the interface between the surfaces of the pins and the surfaces of the pin holes. As the pins in a conventional mold are moved away from the pole, the pins become increasingly prone to spreading, flexing and wear. Thus, while it would be beneficial to reconfigure mold geometry to provide direct gating at the poles and venting at the parting line of the mold, it is highly desirable to achieve such changes without compromising the useful life of the retractable pins.
Some prior attempts to use this approach to solving the problems associated with conventional retractable pin injection molding are described in U.S. Pat. No. 5,147,657 to Giza, which is incorporated by reference in its entirety, Du Pont application 88-366575 and Bridgestone application JP 09000661 (95-176708). While these references describe remedies for some of the limitations of conventional retractable pin injection molding, each also has certain inherent drawbacks not found in the present invention. For instance, while these references permit relocation of the retractable pins to locations away from the poles of the ball, the potential for accelerated wear on the pins and pinholes in the mold plates due to non-axial forces applied to the pins can be considerable depending upon the angle between the direction in which pins operate and the direction of the forces applied to the pins. In addition, these reconfigured golf ball molds have utilized complex, bulky mechanisms to engage and disengage the pins in a direction normal to the point of contact. Thus, each of these prior solutions to the problems encountered by conventional retractable pin injection molding provides only limited flexibility in pin placement and adds considerable bulk and complexity to the golf ball mold.
The present invention is directed to the formation of an intermediate layer or cover layer over a golf ball core by an injection molding process or injection/compression molding process. In particular, the injection mold utilizes a ring gear assembly having a cogwheel 18 with cam inserts 20 that cause the retractable pins to engage with a ball core 22 in a direction approximately perpendicular to the plane tangential to the point of contact on the ball core by the pins. Because the ring gear assembly will activate retractable pins 16 approximately radially inward toward the ball center, i.e. in a direction approximately normal to the tangential line of contact with the ball, it is expected that problems of wear, periodic inspection and maintenance of the pins and pin holes that are associated with conventional retractable pin injection molding will be significantly reduced. Use of the ring gear assembly also allows the retractable pins to be positioned nearly anywhere in the mold, preferably away from the poles of the golf ball so that sufficient space is provided at the poles for injection of material. As discussed more fully below, relocating the retractable pins away from the poles of the golf ball mold overcomes several limitations found in conventional golf ball injection mold configurations.
The present invention remedies the inherent problems found in conventional multiple gate retractable pin golf ball injection molds by utilizing a compact mechanism to engage and disengage the pins in a direction approximately normal to the tangent of the point of contact on the ball surface through application of forces that differ only slightly from the longitudinal axis of the pin. Because the retractable pins operate in a direction approximately normal to the point of contact with the ball surface, the faces of the pins 24 that contact the ball could be easily and inexpensively shaped to securely grip the ball core when engaged and to correspond to the inner wall of the mold cavity when retracted. For instance, when the injection mold is for forming an intermediate layer of the golf ball, the faces of the retractable pins may be flat or readily shaped to correspond to the curvature of the mold wall. When the mold is for forming a cover layer for a dimpled ball, the faces of the retractable pins may be shaped with a full radius to correspond to a dimple on the ball. The present invention also has the added advantage of allowing retractable pins 16 to be positioned in nearly any location on the ball while imparting minimal non-axial forces during operation. Thus, the effect of wear due to non-axial forces is minimal regardless of the position of the pins. The ability to reposition the retractable pins to virtually any location of the mold also allows the pins to be positioned away from the poles of the mold, thereby permitting pole gating and parting line venting. In turn, injection of layer material at the poles of the ball allows for venting of trapped air and gasses to occur at the parting line between the mold plates, i.e. near the equator of the mold cavity, where the trapped air and gasses may be vented more quickly through a greater area. Thus, cycle times can be improved.
One embodiment of the present invention is a retractable pin assembly having a retractable pin 16 with one end 24 positioned inside a mold cavity and another end 26 outside the mold cavity. A cam 20 affixed to cogwheel 18 is positioned outside the mold cavity so that the cam contacts the outside end 26 of retractable pin 16 and causes the pin to move in or out of the mold as the cogwheel turns.
In one embodiment, the inside end 24 of retractable pin 16 has a shape substantially similar to that of a golf ball dimple, while in another embodiment inside end 24 of retractable pin 16 is shaped substantially in conformance with the curvature of the mold cavity.
In one embodiment, the surface of the cam in contact with the outside end of the retractable pin is essentially planar. More specifically, in one embodiment the planar surface of the cam is inclined at an angle xcex8 less than 15 degrees. In yet another embodiment, the retractable pin assembly includes a spring for retracting the pins from the mold.
Another embodiment of the present invention involves a golf ball injection mold having a first and second mold plate, each having a substantially hemispherical inner surface such that when the mold plates are joined they form a substantially spherical cavity. In addition, a plurality of retractable pins extend from the inner surface of the spherical cavity to the outer surface of a golf ball core placed inside the cavity during the molding process. Each retractable pin travels in a direction approximately normal to the surface of the golf ball core through operation of slidable cams.
In one embodiment, the retractable pins are positioned to contact the ball core at an angle between about 10 degrees to about 80 degrees from the pole-to-pole axis of the golf ball mold. In another embodiment, the pins contact the ball core at an angle between about 30 degrees to about 60 degrees.
One embodiment further includes injection gates positioned at the poles of the mold plates. In another embodiment, the injection gates are connected to a hot runner system. Yet another embodiment further includes at least one vent located near the parting line of the golf ball mold.
In another embodiment, the slidable cams are affixed to at least one cogwheel positioned to rotate about an axis approximately coincident with the pole-to-pole axis of the golf ball mold. In yet another embodiment, the angle between the surface of the sliding cams and the retractable pins is between about 80 degrees to about 100 degrees. And in another embodiment the angle is approximately 90 degrees.